Acute hyperthermia has been demonstrated to profoundly affect a variety of biochemical systems in the newborn rat, whereas mature animals appear not to be affected. In particular, hyperthermia causes complete loss of brain polyribosomes due to disaggregation. This results in a profound inhibition of in vivo protein synthesis and a total loss of ornithine decarboxylase, the rate-limiting enzyme for polyamine biosynthesis. Recovery from heat shock is characterized by an overshoot-increased ornithine decarboxylase activity; this can be blocked by pharmacological agents or by prior heat exposure indicating receptor desensitization.